refactor: 废弃 rate_multiplier 字段，改进成本倍率回退逻辑

- 在前后端模型中为 rate_multiplier 添加 @deprecated 标记
- 修复 rate_multipliers 回退逻辑：当 rate_multipliers 存在但未配置当前格式时使用 1.0
- ModelMappingsTab: 删除映射后自动保存，简化空状态提示
- ModelManagement: 添加 refreshModel 事件在映射更新后刷新模型数据
- CacheAwareScheduler: 添加映射匹配调试日志，移除 candidate_models 限制

frontend/src/features/models/components/ModelMappingsTab.vue

@@ -155,15 +155,12 @@
     <!-- 空状态 -->
     <div
       v-else
-      class="text-center py-8"
+      class="text-center py-32"
     >
       <GitMerge class="w-10 h-10 mx-auto text-muted-foreground/30 mb-3" />
       <p class="text-sm text-muted-foreground">
         暂无映射规则
       </p>
-      <p class="text-xs text-muted-foreground mt-1">
-        添加映射可匹配 Provider Key 白名单中的模型
-      </p>
     </div>
   </Card>
 </template>
@@ -462,14 +459,15 @@ function addMapping() {
   expandedIndex.value = localMappings.value.length - 1
 }
 
-function removeMapping(index: number) {
+async function removeMapping(index: number) {
   localMappings.value.splice(index, 1)
-  isDirty.value = true
   if (expandedIndex.value === index) {
     expandedIndex.value = null
   } else if (expandedIndex.value !== null && expandedIndex.value > index) {
     expandedIndex.value--
   }
+  // 删除后自动保存
+  await saveMappings()
 }
 
 async function saveMappings() {